Heavy top quark, fermion mixings and all that

An attempt is made to incorporate the observed structure of the quark and lepton masses and mixings within the framework of SO(10) grand unification supplemented by a Peccei-Quinn U(1) symmetry that also distinguishes the three chiral families. All dimensionless couplings are assumed to be of order unity. The hierarchical structure among the charged fermion masses, including the t-b splitting, gets related to the 'small' (∝10-2) ratio Mc/Mx (or its powers), where Mc(Mx) denote the SU(4)c (SO(10)) breaking scales. An upper bound on the top quark mass is obtained wich, for central values of the parameters, is more restrictive than other well known bounds. The mixing angles in the charged lepton sector are related via the SU(4)c symmetry to the quark mixing angles. One finds that asymptotically, θμe ≅ (ms/mμ)θ12 θτμ ≅ θ23, and θτe ≅ θ13. In the neutral sector the tau neutrino may be cosmologically significant, whereas νe and νμ can play a role in the MSW effect. (orig.)